Control of Microbiome Composition and AgrichemicalRemediation in Woodchip Bioreactors by Hydraulic Retention Time

Woodchips in water treatment bioreactors provide a nutritional surface for microbial communities that metabolize fertilizers and pesticides. To better understand their bioremediation capacity, we treated them using simulated water carrying nutrients (nitrate and phosphate) and 5 physicochemically distinct pesticides through 20 two-stage systems, each comprised of a woodchip bioreactor and a heat-expanded shale sorption matrix, operated at hydraulic retention times (HRTs) of 4, 24, or 48 h for nearly 3 months. The woodchip bioreactors effectively removed nitrate (90–96%) and phosphate (40–64%), particularly at longer HRTs due to the enrichment of wood-decomposing fungi and nitrate-reducing bacteria. Pesticide addition did not affect nutrient removal, as nitrate-reducing taxa known to tolerate or degrade pesticides were enriched. The pesticide removal efficiency of the bioreactors correlated positively with HRT length (acephate, 2–8%; triflumizole, 75–90%; isoxaben, 49–87%; prodiamine, 50–77%; and bifenthrin, 30–40%). Furthermore, longer HRTs also stimulated phosphate and isoxaben removal in the expanded shale stage. These findings underscore the benefits of HRT operation on bioreactor acclimation and remediation capacity and identify 24 h as the HRT that treats the largest volumes of water without compromising contaminant removal efficiency.